Correspondence — Br. A. von Koenen. 103 



Mr. Forbes has treated the results of various assays (made for com- 

 mercial purposes) which I quoted, as if intended by me as evidence of 

 this mineral being identical in composition with that from the Fox-dale 

 mine, which would have been absurd. The figures were given 

 solely for the purpose of showing that this ore contains certain 

 quantities of silver ; and I specially stated that I knew of no 

 analysis having been made of it. Mr. Forbes will notice, if he 

 refers to my letter, that I did not use the term polytelite at all, 

 Glocker having proposed this name in 1847,^ for a mineral analysed 

 by Eammelsberg in 1846,^ which not only contained between 6 and 

 6 per cent, of silver, but also from 36 to 38 per cent, of lead, with 

 only 0-32 per cent, of copper (and which has been regarded by some 

 mineralogists as an argentiferous hournonite). I do not believe the 

 Cornish ore contains any lead. 



The difference of opinion appears to arise from the question, as to 

 what constitutes a silver-fahlerz ; but I had not, nor have T now, the 

 least intention to enter upon a discussion respecting tetrahedrite, and 

 its many varieties, considerable difference of opinion existing as to 

 the precise limits of the latter. It is quite possible that this ore 

 (which is worked and sold in Cornwall as a silver and copper ore) 

 may be an argentiferous tetrahedrite only ; and that is precisely the 

 point I hoped to induce Mr. Forbes to determine by analysis, and 

 hence my letter. Thos. Davies. 



P.S. — Since writing the above I have been favoured with a letter 

 from Prof. A. H, Church, of the Eoyal Agricultural College, Cirencester^ 

 in which he says : — " I have found in one of my laboratory books the 

 determinations of silver in Cornish fahlerz to which I alluded in 

 conversation with you some time ago. They were made in August, 

 1865, for the purpose of ascertaining the value of the ore raised from 

 the Silver-vein mine near Lostwithiel. The following were the 

 results : *73^|o Silver in a mixed sample of ore in coarse powder. 

 7-23^|o Silver in a crystallized fragment of fahlerz, having the density 

 4-85. 10-45^|o Silver in another crystalline mass." — T.D. 



THE BELGIAN TERTIAMES. 

 Sir, — In the December number (p. 565), Mr. Godwin-Austen 

 protests against the observations which I made on his paper on the 

 Belgian Tertiaries, in my article in the Geological Magazine for 

 November last (p. 501). With regard to my objections, I can only 

 assure him that I wrote them down in order to remove mistakes, 

 and without the slightest intention of personally offending him. 

 Mr. Godwin-Austen gives a list of fossils from the Cassel-beds 

 (Upper Oligocene) in order to corroborate his opinion on their 

 relative age. I am not aware now where this list is taken from, 

 but that is of no consequence : but I must assure him that nearly all 

 the names there cited are erroneous, according to the works of 

 Sandberger (on the Mayence Basin), of Beyrich (Norddeutsche 



^ " Generum et Specierum Mineralium Synopsis," by E. F. Glocker, Halle, Saxony, 

 1847, 8vo., p. 31. 

 2 " PoggendorfTs Annalen," vol. Ixviii. 1846, p. 516. 



